An energy harvester which generates low power, such as an electret-based vibration energy harvester, provides high-voltage power; the power supply circuit is known to convert this high-voltage power into low voltage power for operating a load (i.e., an electronic circuit).
This kind of power supply circuit temporarily stores the energy from the energy harvester in a low-capacity capacitor on the input side, and uses a buck converter to charge a high-capacity capacitor on the output side with the energy stored in the input-side capacitor. However, if the power needed to drive the buck converter is taken from the input-side capacitor, the power supplied from the energy harvester to the power supply circuit is needlessly consumed since it is high-voltage power. Therefore, it is suggested that the energy for driving the buck converter should come from the output-side capacitor.
More specifically, it is proposed to provide a first fixed current circuit, and a second fixed current circuit, wherein the first fixed current circuit generates a fixed current from the voltage of the input-side capacitor, and the second fixed current circuit generates a fixed current from the voltage of the output-side capacitor. If the output-side capacitor provides low voltage, the buck converter is driven using current from the first fixed current circuit, and if the output-side capacitor provides high voltage, the buck converter is driven using current from the second fixed current circuit (see Patent Literature 1). The output-side capacitor and the input-side capacitor are charged simultaneously until the voltage in the input-side capacitor reaches a predetermined voltage. When the voltage in the input-side capacitor reaches the predetermined voltage, the output-side capacitor is disconnected from the input-side capacitor and the buck converter is driven using the voltage from the output-side capacitor (see Non-Patent Document 1).